1. Field of the Invention
The present invention relates to filters used with radio frequency tuners and specifically to tunable tracking filters.
2. Prior Art
Broadband receivers such as TV and cable tuners that are built with discrete components often incorporate tracking filters that operate on the radio frequency (RF) input signal prior to conversion from RF to intermediate frequency (IF) or baseband. The filters are tune-able such that they can be set to have their center frequencies at the desired channel. This has several benefits:
1. Superior distortion performance: Jammers are attenuated before they reach subsequent circuitry.
2. Image rejection: The subsequent mixer down converts two signals to the same IF frequency, the desired channel as well as the undesired image channel at a frequency above or below the desired channel frequency. If ‘IF’ denotes the IF frequency, ‘CH’ denotes the desired channel frequency, and ‘IM’ denotes the image channel frequency, then IF=CH−LO but also IF=LO−IM. Thus both CH and IM convert to the same IF output frequency, resulting in possible corruption of the desired channel output. The tracking filter can be used to help suppress the image channel.
3. Harmonic suppression: In addition to LO-RF, the subsequent mixer also to some extent down-converts signal at the harmonics of the LO, i.e. 2LO-RF, 3LO-RF, etc. The filtering suppresses signals that might reside at multiples of the LO and thereby helps in preventing contamination of the down-converted desired channel.
Prior art techniques implement RF tracking filters with discrete components. In discrete circuits, the tracking filters are built using very low loss, high Q, passive components as well as varactors, which usually achieve the necessary linearity by utilizing a very high tuning voltage. With this method, very sharp low loss filtering can be achieved. This type of component choice is normally not available in integrated circuit technologies and therefore the circuit techniques are not applicable to these types of receivers implemented in integrated circuit technology.
Christensen, U.S. Pat. No. 6,778,023 entitled “Tunable filter and method of tuning a filter”, issued Aug. 17, 2004, incorporated herein by reference, discloses a bandpass filter tuned by converting the filter in an oscillator and tuning the oscillator frequency using conventional tuning techniques such as tuning a varactor and measuring the oscillator frequency.
Birleson, U.S. Pat. No. 6,714,776 entitled “System and method for an image rejecting single conversion tuner with phase error correction” issued Mar. 30, 2004, incorporated herein by reference, discloses a single conversion tuner with tuned bandpass and image reject notch tracking filters to reject out of band and image signals.
Vorenkamp et al., U.S. Pat. No. 6,285,865 entitled “System and method for on-chip filter tuning”, issued Sep. 4, 2001, incorporated herein by reference, discloses various aspects of one approach to a tunable LC filter using a dummy circuit and an active circuit. Vorenkamp examines phase transition at the center of the filter band to tune the filter.
Spencer, U.S. Pat. No. 6,823,292 entitled “unable Filter”, issued Nov. 23, 2004, incorporated herein by reference, discloses a tunable filter for a single chip radio receiver. A bank of switchable passive capacitors connect in parallel to an inductor and a varactor to tune the center frequency of filter.